Abstract
The objectives of this research are the preparation and characterization of carbon nanomaterials reinforced nanocomposite bipolar plates for use in proton exchange membrane fuel cells (PEMFCs). There are four parts in this dissertation. The first part of this research is to prepare novel metal mesh hybrid polymer composite bipolar plates for PEMFCs via inserting a copper or aluminum mesh in polymer composites. The composition of polymer composites consists of 70 wt% graphite powder and 0-2 wt% modified multi-walled carbon nanotubes (m-MWCNTs). Results indicate that the in-plane electrical conductivity of m-MWCNTs/polymer composite bipolar plates increased 312 % from 156 S cm-1 (0 wt% MWCNT) to 643 S cm-1 (with 1 wt% MWCNT) (D.O.E target ＞ 100 S cm-1). The bulk thermal conductivities of the copper and aluminum mesh hybrid polymer composite bipolar plates (abbreviated as Cu-HPBP and Al-HPBP) increased from 27.2 W m-1 K-1 to 30.0 W m-1K-1 and 30.4 W m-1 K-1, corresponding to the enhancements of 10.3% and 11.8%, respectively. The through-plane conductivities decreased from 37.8 S cm-1 to 36.7 S cm-1 for Cu-HPBP and 22.9 S cm-1 for Al-HPBP. Furthermore, the current and power densities of a single fuel cell using copper or aluminum mesh hybrid polymer composite bipolar plates are more stable than those of using neat polymer composite bipolar plates, especially in the ohmic over-potential region of the polarization curves of single fuel cell tests. The single cell performance can maintain at 0.663 A cm-2 and 0.282 W cm-2. The overall performance confirms that the metal mesh hybrid polymer composite bipolar plates prepared in this study are promising for PEMFC application. The second part of this study investigates the flowability of the bulk molding compound (BMC) on composite bipolar plates containing graphite content from 70 to 80 wt% with different graphite sizes. A small quantity (from 0.25 to 2 wt%) of multi-walled carbon nanotubes (MWCNT) is also added. Results show that the flowability of BMC material decreases with decreasing graphite size and with increasing graphite content. The BMC material containing large size graphite (177-125 μm) entirely exhibits a relatively higher flowability within the analysis graphite contents, in the range of 73.3 cm to 11.3 cm (in spiral flow mold), compared to the small size (74-45 μm), in which flowability is in the range of 40.3 cm to 6.67 cm. Further adding MWCNT causes decreased flowability of the BMC material especially when the percolated networking structure is formed through the curing of resin. Therefore, with flowability below 10 cm, the formability of a large area (300 mm×300 mm×3 mm) or thin (100 mm×100 mm×0.5 mm) composite bipolar plate shows a large area of surface porosity or visible defects. Results indicate that the flowability of the thermoset-based BMC material is an important design parameter to fabricate cost-effective, large, or thin composite bipolar plates. The third part of this research presents a novel nanocomposite bipolar plate (BP) reinforced by graphene at a low weight fraction, and compares the properties of this novel nanocomposite BP with those containing various weight fractions of multi-wall carbon nanotubes (MWCNT) (0.2, 0.5, and 1 phr, parts per hundred parts of resin). Adding only 0.2 phr of graphene as reinforcement remarkably enhanced the thermal, mechanical, and electrical properties of the nanocomposite BP. The coefficient of thermal expansion (CTE, α) of nanocomposite BP below the glass transition temperature (α1) decreased from 49.7 μm m-1℃-1 to 26.9 μm m-1℃-1 and the CTE above Tg (α2) decreased from 119.2 μm m-1℃-1 to 55.2 μm m-1℃-1. Thermal conductivity increased from 18.4 W m-1 K-1 to 27.2 W m-1 K-1. The flexural strength increased from 28.0 MPa to 49.2 MPa. The in-plane electrical conductivity increased from 155.7 S cm-1 to 286.4 S cm-1. The enhancement percentages of these properties are 47.8 %, 75.7 %, and 83.9 %, respectively, which are much higher than those of the original composite BP. The maximum current densities and power densities for PEMFC single cell performance with 0.2 phr graphene are increased from 2.06 A cm-2 to 2.37 A cm-2 and from 0.650 W cm-2 to 0.788 W cm-2, corresponding to a enhancement of 15.0% and 21.2%, respectively. These results indicate that using graphene as reinforcement in the preparation of nanocomposite BP is effective in terms of cost and performance, due to the low cost of the raw material, graphite, and to the fact that a lower loading of graphene than of MWCNT can obtain the same performance. Moreover, this novel multi-functional nanocomposite BP possesses great potential in PEMFCs applications. The forth part of this research investigates a covalent functionalization of graphene by free-radical modification. Maleic anhydride modified poly (oxyalkylene)-amines with different molecular weights (POA400 and POA2000) were grafted onto the surface of graphene, forming Graphene/MA-POA400 and Graphene/MA-POA2000 in order to increase the compatibility between grapheme and organic phase. Results investigated by XPS, TEM, Raman, and TGA all confirm that these molecules were covalently grafted to the surface of graphene. The functionalized graphene exhibited higher solubility than that of the pristine graphene in organic solvents and showed well dispersion in the vinyl ester matrix. The compatibility increased in the order of Graphene/MA-POA2000 > Graphene/MA-POA400 > Graphene. Furthermore, this study also investigated the mechanical, electrical and single fuel cell properties of functionalized graphene nanocomposite bipolar plates for use in PEMFCs. The coefficient of thermal expansion (CTE, α) of nanocomposite BP containing 0.2 phr grapheme/MA-POA2000 below the glass transition temperature, α1, decreased from 49.7 μm m-1℃-1 to 23.9 μm m-1℃-1 and the CTE above Tg, α2, decreased from 119.2 μm m-1℃-1 to 35.0 μm m-1℃-1. The flexural strength increased from to 28.0 MPa to 58.5 MPa. The in-plane electrical conductivity increased from 155.7 S cm-1 to 631.4 S cm-1. The enhancement percentages of these properties are 109.0 % and 305.5 %, respectively, which are much higher than that of the original composite BP. The maximum current densities and power densities for PEMFC single cell performance with 0.2 phr grapheme/MA-POA2000 are increased from 2.06 A cm-2 to 2.50 A cm-2 and from 0.650 W cm-2 to 0.906 W cm-2, corresponding to the enhancements of 21.4 % and 39.4 %, respectively. The overall performance confirms the grapheme/MA-POA2000 nanocomposite bipolar plates prepared in this study are suitable for PEMFC application.